Use correct coordinates in OpenMRG example dataset

[cchwala]

With the current OpenMRG example data there are still thing like this that we need to do:

# Fix error where lat/lon was set to variable
ds_rad = ds_rad.set_coords(["longitudes", "latitudes"])

# Fix naming errors
ds_gauges = ds_gauges.rename({"station_id": "id"})
ds_gauges_smhi = ds_gauges_smhi.rename({"station_id": "id"})

ds_rad = ds_rad.rename({"longitudes": "lon", "latitudes": "lat"})

Would be great if the dataset here can be updated. When we do this, we also need to update its usage in the poligrain
example notebooks and in some tests... 🙈

@eoydvin Maybe you find time for this during the next days/weeks. If not, we can discuss during the next dev meeting.

[cchwala]

In #3 this was done correctly, but the existing '5min_2h' data needs to be fixed.